$300 and major mods sounds like a headache to me. Juss' sayin.
You could get the CMQFR and a set of nice keys for less. And it'd do 90% of what the kinesis does. Up to you to decide whether that last 10% is worth the squeeze.
I don't think that's really a fair comparison at all. Trying to decide between a Kinesis and a Quickfire is one of those apples vs oranges things, we're not talking about minor differences. And not to attack swagpiratex or anything, but I don't think that's a statement coming from someone who's tried a Kinesis, though I don't know for sure.
I don't get terrible pain or anything while using a flat keyboard, but after awhile I may feel a little something in my wrist, not enough that I would be worried about it, just a tiny bit of soreness from holding my wrist at the angle needed for a keyboard. With the Kinesis I never get this feeling, it's extremely comfortable for use.
Also the Kinesis lets you do several things that flat keyboards do not, such as stay near the main key cluster all the time, because the arrows and six pack are reachable from your resting position. You can also reprogram the keys and move them to anywhere you please, which I actually make pretty good use of.
If you have a good reason to consider the Kinesis (worried about ergonomics or like the efficient button placement), then I would easily recommend it. If you are just getting a mechanical keyboard because you think they're cool (since this is your first one), then a quickfire would probably be fine.